File Replication

Jim McKinstry

Today, it's a given that UNIX systems need to share data across a network. A less common requirement is to have copies of data on multiple servers. The most common solution for sharing data is NFS. However, there is no widely accepted way to provide copies of data across the network. One possible solution is to replicate the data periodically to meet your needs.

While NFS provides current, real-time access to data on another server, replicated data will lag behind the original by minutes or hours. This is not a problem since replicated data is usually not used for real-time processing. Combined with Replication Server from Sybase, I have seen file replication provide an effective way to off-load defect correction tasks and report generation to a non-production server allowing the production server to run more efficiently. File replication is also ideal for off-loading production data that has been archived. Most importantly, critical system configuration files (vfstab, crontab, volume configuration, etc.) can be replicated to other servers so that the information is recoverable in case of a failure.

Originally, this file replication solution was written in C and used pipes. An associate hacked the solution out of a book and tweaked it to work on our systems. When it stopped working, I was asked to fix the problem. After a couple of days of debugging, I decided to rewrite the solution so that people with minimal coding experience would be able to maintain it. This version of the software has been running successfully in a production shop for more than 6 months and should be very easy to maintain. It is much less complex, much more reliable, and actually faster than the original C implementation.